This invention relates to avionics communication equipment, and more particularly to phase locking apparatus for channel selection and tuning in avionics communication systems.
In accordance with standard procedures and federal law, rigid specifications are established for communication equipment. For airborne communication apparatus, a 720 channel band, of 25 KHz width is specified, spaced in the frequency band 118 through 135.975 MHz. Communication occurs between an airborne person and others by selecting an available one of the 720 channels and communicating thereon.
Within these established constraints, there exists a need for effective tuning and channel selection apparatus whereby the user may, by manually selecting a channel for communication, be able promptly and effectively to communicate thereon. Moreover, once a channel allocation is made, the apparatus must be capable of maintaining tight frequency supervision over the allocated channel, to avoid such problems as loss of connection between the communicating parties, co-channel interference and crosstalk, and the like.
General objects of the present invention include providing avionics communication equipment which meets relevant FCC and FAA specifications, and furthermore functions in a convenient effective manner within the above requirements.
One approach to frequency supervision and control is the use of phase locked loops. Typically, these loops feature phase or frequency comparison of the signal to be synchronized with a reference signal, and utilization of the difference noted to adjust the frequency of a voltage controlled oscillator. The oscillator output in turn constitutes the signal to be snythesized.
It is an object of the present invention to provide phase locking apparatus adapted for utilization in avionics communication gear. Such application, however, is not straightforward because of difficulties attendant to the wide frequency spectrum of the communication channels. That is, the narrow band channels spread over a wide, relatively high frequency range make it difficult to apply phase locking techniques. More specifically, the avionics band of 118 through 135.975 MHz, with 25 KHz channel bands, makes straightforward application of phase locking techniques difficult, because use of plural, even frequency division factors conventionally involves apparatus of considerable complexity.
It is accordingly a further object of the present invention to apply phase locking techniques to multiple channel, relatively high frequency communication systems, and specifically to the avionics band.